1. Field of the Invention
The invention relates to a differential amplification circuit compensating for an offset voltage from a variance in resistances, and a manufacturing method.
2. Description of the Related Art
In the past, a current detection device including a shunt resistor that detects a current flowing into a battery, an amplifier that amplifies a voltage across the terminals of the shunt resistor, and a switch that is used to strap two input terminals of the amplifier so as to temporary bring an input voltage to a zero or near zero value as been proposed. Refer, for example, to JP-A-2006-64627 where such an amplifier encompassing a differential amplification circuit is described.
In a prior art current detection device, the shunt resistor has the terminals thereof temporarily strapped by the switch in order to measure an offset voltage of the amplifier, and the offset voltage is then subtracted and eliminated from a remaining battery capacity.
In recent years, sensors for detecting a battery voltage, a battery current, or a temperature have been mounted in a vehicle for the purpose of sensing the state of an automobile battery. A current sensor has come to be used to detect a current and the detection methods include a magnetic detection method using a Hall element, and a resistive method. A resistive current sensor uses a differential amplification circuit to detect a potential difference across the terminals of a detection resistor, and has the advantage that the detection resistive element itself is, in principle, devoid of an offset and that the temperature dependency of sensitivity can be reduced by employing a resistor whose temperature coefficient is small.
However, when a device such as the device described in JP-A-2006-64627 is mounted in a vehicle, since an automobile battery current ranges from about 500 A to at most about 1000 A, and since a shunt resistor has to offer a resistance of about 0.1 mΩ, a resistance of the switch to be used to strap the terminals of the shunt resistor is required to be 1/100 or less of the resistance of the shunt resistor, that is, about 1μΩ or less for bringing an error to 1% or less. The adoption of a switch satisfying the condition is unfeasible. However, if a correction means is not adopted, various microscopic resistances of wirings exist with the sensors attached. Consequently, the potential at the entire shunt resistor that is the detection resistor fluctuates due to a potential drop. Experimental study has revealed that, eventually, an error which is potential-dependent and derived from a variance in resistances in the differential amplification circuit poses a problem as will be described herein below with reference to FIG. 24.
As shown, a load 61 and an electronic control unit (ECU) 62 are connected to an automobile battery 60. The load 61 is connected to a body ground 63. The body ground 63 is connected to the battery 60 via various wiring resistors, for example, a fastening bolt contact resistor 64, a harness resistor 65, a fastening bolt contact resistor 66, a shunt resistor 67 for detecting a battery current, and a battery post contact resistor 68.
A differential amplification circuit 69 for detecting a battery current is connected to the terminals of the shunt resistor 67, and a differential amplification circuit 70 for detecting a battery voltage is connected to the terminals of the battery 60. The ECU 62 directly or indirectly monitors the battery current and battery voltage using the outputs of the differential amplification circuits 69 and 70. Moreover, the grounds of the differential amplification circuits 69 and 70, or more particularly, the grounds of operational amplifiers included in the differential amplification circuits 69 and 70 respectively are connected to the body ground 63 that is used in common with the ECU 62 and load 61.
However, as shown in FIG. 24, between the body ground 63 and battery 60, the wiring resistors including the fastening bolt contact resistor 64 exists in addition to the shunt resistor 67. Consequently, when a discharge current flows from the body ground 63 to the shunt resistor 67, a voltage drop of several volts occurs in the range from the body ground 63 to the shunt resistor 67 due to the fastening bolt contact resistor 64 and others. In contrast, a potential difference across the terminals of the shunt resistor 67 which is handled as a current detection signal is 100 mV or less.
Namely, the potential at the terminal of the shunt resistor 67 on the side of the body ground 63 is ideally identical to the ground potential at the body ground 63. In reality, since the wiring resistors including the fastening bolt contact resistor 64 cannot be ignored, the potentials at the terminals of the shunt resistor 67 largely vary because of the wiring resistors.
On the other hand, the resistance values of the resistors in the differential amplification circuit 69 vary with respect to design values due to factors such as the precision of a manufacturing apparatus during manufacture of the differential amplification circuit 69. An offset voltage derived from the variance in resistances therefore occurs in the differential amplification circuit 69.
Consequently, when the differential amplification circuit 69 is incorporated in a system shown in FIG. 24, the overall potential at the shunt resistor 67 fluctuates due to the adverse effect of the wiring resistors. Accordingly, the offset voltage of the differential amplification circuit 69 fluctuates. In other words, a potential-dependent error derived from the variance in resistances in the differential amplification circuit 69 occurs in the differential amplification circuit 69. The error causes degradation in the precision of the differential amplification circuit 69. The same applies to the differential amplification circuit 70 for detecting a battery voltage.